1. Field of the Invention
The present invention relates to an orthogonal frequency division multiplexing (OFDM) transmission/receiving system and a block encoding method therefor.
2. Description of the Related Art
OFDM, which is a multiplexed carrier modulation method, stands up very well against multi-path fading and inter-symbol interference, and provides a low signal-to-noise ratio (SNR). In OFDM, data can be reliably transmitted at a high data transmission rate even in a channel where severe temporal dispersion occurs. In particular, the OFDM technique is suitable for the radio communications field, and has been applied to wireless LAN and digital audio or video broadcasting.
However, the OFDM technique has a disadvantage in that, in certain circumstances, a very high peak-to-average power ratio (hereinafter, referred to as a PAR) is obtained upon transmission of an OFDM signal. For instance, when data is transmitted using N subcarriers, a peak power, which is N times as large as an average power, can be obtained, such as in situations were N in-phase signals are summed. The peak power, which is greater than the average power, becomes a non-linear factor and provokes intermodulation between subcarriers. This deteriorates the SNR at a receiver and causing unwanted out-of-band radiation. If a power amplifier or mixer operates with a small back-off, it is not possible to maintain out-of-band power within the limits imposed by telecommunications authorities, and the SNR at the receiver is deteriorated. Therefore, the power amplifier preferably operates with a large back-off to prevent spectral regrowth of an OFDM signal due to the intermodulation between subcarriers and unwanted out-of-band radiation. This means that the amplifier operates ineffectively. Consequently, the cost of transmitters increases.
In order to solve this problem, it is preferable to reduce the PAR by performing fast Fourier transforms (FFT) or an alternative method of changing the phase of a signal or a method of using a block code. In the method of using FFT, a solution for reducing the PAR is searched by adding a block which has a value at only an arbitrary subcarrier in front of a fast Fourier transformer (FFT) and alternating FFT and inverse FFT (IFFT). However, in this method, the same work is repeated on every transmission data to find a value for reducing the PAR. Accordingly, realization of this method is complicated, and temporal delay occurs, so that this method is not suitable for telecommunications systems.
The method of changing the phase is to appropriately convert the phases of the N signals to be transmitted in order to prevent the N signals from having the same phase. This phase changing method includes complementary codes using Golay codes and Reed-Muller codes. The complementary codes use a phase shifter which is expressed as an exponential function, in order to convert the phases of the N signals into a set of phase codes. However, the phase shifter has a problem in that it is complicated to realize in hardware.
The block code method has a disadvantage in that an encoder and a decoder are large due to a codebook. Also, only a block code for binary modulation data is considered in this method, so the data transmission rate is low.
To solve the above problems, an objective of the present invention is to provide an orthogonal frequency division multiplexing (OFDM) transmission system for encoding binary data having a predetermined length into Q-ary data which is expressed in a pair of two bits, and modulating the encoded binary data, and an OFDM receiving system for demodulating and decoding a received OFDM signal, and a block encoding method for encoding the binary data into Q-ary data.
To achieve the above objective, the present invention provides an orthogonal frequency division multiplexing (OFDM) transmission system including: a block encoder for encoding binary data of U length into V-ary data expressed in a group of n bits according to a predetermined mapping rule; a serial-to-parallel converter for converting the V-ary data into parallel data; a V-ary modulator for V-ary modulating V-ary data received in parallel to generate an orthogonal frequency division multiplexing (OFDM) symbol having U sub-symbols; and a transmitter for post-processing the OFDM symbol so that the OFDM symbol is suitable for telecommunications and transmitting the resultant OFDM symbol.
To achieve the above objective, the present invention provides an orthogonal frequency division multiplexing (OFDM) receiving system including: a preprocessor for pre-processing an OFDM symbol transmitted via U carrier waves so that the OFDM symbol is suitable for demodulation; a V-ary demodulator for V-ary demodulating the preprocessed OFDM signal to generate V-ary data expressed in a group of n bits; a parallel-to-serial converter for converting the V-ary data into serial data; and a block decoder for decoding serial V-ary data into binary data of U length according to a predetermined mapping rule.
To achieve the above objective, the present invention provides a block ending method for OFDM transmission, including: generating V-ary data expressed in a group of n bits; generating an OFDM symbol by modulating and inverse-fast-Fourier-transforming the V-ary data; determining whether the OFDM symbol satisfies a predetermined condition, and if the OFDM symbol satisfies the predetermined condition, classifying the OFDM symbol as a candidate codeword; extracting as many codewords as the number of receivable binary data from candidate codewords, in sequence of codewords having a small bit change; and forming a codebook by matching the extracted codewords to each receivable binary data.